This is a broad-gauged, programmatic effort on the part of several investigators to assess the relative roles of cognitive, motivational and biological factors in the intellectual functioning of retarded individuals. With regard to cognitive factors, a variety of orthodoxy learning tasks, concept formation tasks, measures of linguistic competence, and standard intelligence tests will be investigated in order to compare the intellectual functioning of retarded children with that of intellectually normal individuals of comparable MAs and /or CAs. With regard to motivational factors, attention will be focused on the effects of experiences which are common for many retarded persons and which appear to influence their behavior, such as social deprivation, institutionalization, and frequent failure, which in turn give rise to such motivational effects as strong positive and negative reaction tendencies in interacting with adults, low expectancy of success, and non-optimal reinforcement hierarchies. These two types of factors and their interaction will be investigated in institutionalized and noninstitutionalized retarded persons with varying etiologies, MAs, and CAs, and also in groups of children from different socioeconomic backgrounds who experience different kinds of educational intervention programs. Biological factors will also be investigated with children showing severe developmental disabilities such as autism, severe language impairment, and learning disabilities. These studies aim at elucidating the relationships between biological processes, environmental inputs, and children's competencies. The long-range goal is the discovery of those interventions, both in and out of institutions, which would result in optimizing the performance of every retarded child at whatever intellectual level he might be.